There are many instances where the use of one electrically powered device necessitates the need or desire to use a second device. One such instance exists in the industrial or home workshop. Often, after a power tool is activated, it is desirous to also activate a vacuum or other electrically powered particle collector. Most power tools have a 1¼″ or 2″ dust port for a shop vacuum connection in order to collect sawdust and wood chips. For example, the operator activates a power tool, such as a drill, a miter or table saw, and some time shortly thereafter, the vacuum automatically engages. Once the power tool has been disengaged, the vacuum automatically disengages a short time thereafter. In this way, the operator is freed from the inconvenience of having to activate and deactivate the vacuum before and after the use of a power tool. Similarly, there are other situations where this solution would be desirous. For example, a user, upon activating his television, may desire that his stereo or cable box automatically activate. Similarly, the activation of one's personal computer could be used to automatically engage the associated monitor, printer or other peripheral devices.
Many of the circuits designed to address this problem suffer from one or more shortcomings. For example, some circuits activate a second electrical outlet immediately upon detecting current flow through a first outlet. This can cause circuit breakers to trip due to the large simultaneous startup currents of the two devices. To correct this, some circuits utilize an analog delay mechanism, such as an RC time constant to offset the activation times of the two devices. While this alleviates the problems associated with simultaneous activation, such circuits are inflexible, only capable of a predetermined delay, which is only alterable through changes to the components on the circuit board. Also, analog components, such as op-amps and the like, can be less reliable than digital circuitry.
The present invention overcomes the shortcomings of the prior art through the use of a digital microcontroller controlled circuit, with increased reliability and which allows the turn-on and turn-off delays to be easily programmed and modified.